To a hearing aid wearer, it is desirable that a hearing aid can be worn under any circumstances. For example, in the case where the wearer sweats as a result of a proper amount of exercise, in the case where the wearer bathes at an unfamiliar place such as a sports center, or in other cases, it is desirable that the wearer should not take off his/her hearing aid because of a need for sufficiently obtaining information through his/her acoustic sense. In such circumstances, however, the hearing aid wearer presently takes off his/her hearing aid or wears it while fretting about whether the hearing aid fails due to the entry of sweat or water.
In such a situation, as a waterproof structure for an ear-hang type hearing aid, a structure has been known in which a waterproof film holding member that holds a nonporous waterproof film is arranged in front of the sound inlet of a microphone, by which the enclosed state of a microphone chamber formed by the waterproof film holding member and a microphone case is improved (for example, refer to Patent Document 1).
Also, as a waterproof structure for an ear-insertion type hearing aid, a structure has been known in which a holed cap can be attached to a sound outlet connecting part of a hearing aid, and a microporous film made of non-tacky polytetrafluoroethylene is provided in the cap so that sounds can be transmitted easily and also earwax, moisture, sweat, and the like can be prevented from entering into the hearing aid from the auditory canal (for example, refer to Patent Document 2).
Further, a hearing aid has been known in which, a protective device using a nonporous diaphragm, which is formed of a material having a high sound propagation property such as titanium of 0.01 mm or thinner, in place of the microporous film is provided at a sound inlet opening and a sound outlet opening (for example, refer to Patent Document 3).
Patent Document 1: Japanese Patent No. 2869505
Patent Document 2: European Patent No. 0310866
Patent Document 3: Japanese Patent Application Publication No. 10-126897
However, in the waterproof structure of the hearing aid described in Patent Documents 1 and 3, the sound opening is covered with the nonporous waterproof film or the nonporous diaphragm, so that a sound entry pathway into the microphone (microphone chamber) and a sound exit pathway from an earphone (earphone chamber) become in an hermetically-sealed state, respectively. In such an enclosed state, if the air pressure or temperature on the outside of the hearing aid changes, a difference in air pressure arises between the inside and the outside of the enclosed space, and a pressure caused by this difference in air pressure acts on the waterproof film etc., by which a high tension is produced on the waterproof film. As the result, the acoustic impedance of the waterproof film etc. increases suddenly, and the attenuation of sound pressure caused by the waterproof film etc. increases, which poses a problem in that the sensitivity as a hearing aid decreases greatly.
Also, if the microporous film is used as described in Patent Document 2, the difference in air pressure does not arise, but the hearing aid of this type has a disadvantage that the hole in the film is easily clogged with earwax etc. Also, the microporous material formed of polytetrafluoroethylene has a larger specific gravity than a nonporous polyurethane elastomer material, and for this microporous material, if the film is made thin, the waterproof performance thereof decreases, so that it is difficult to sufficiently decrease the surface density of the film, which poses a problem in that it is difficult to sufficiently decrease the acoustic impedance of the film.
The acoustic impedance of film is substantially determined by the acoustic stiffness thereof in a frequency zone lower than the first resonance frequency of the film. The acoustic stiffness of a circular film is proportional to the tension of the film and inversely proportional to the biquadrate of the film diameter. Especially in the case of the ear-insertion type hearing aid, the diameter of waterproof film is about 2 mm from the viewpoint of design. If the film diameter decreases, the variation in film acoustic impedance with respect to the change in film tension increases suddenly. Thus, for the waterproof hearing aid, it is important to adjust the air pressure on the inside and the outside of the film so as to be in equilibrium to prevent the film tension from changing.